1889.] On Variation. 1085 
came an astonishing increase in numbers, and this continued up 
to Devonian time. 
With the genus Orthoceras there is a condition of affairs ap- 
proaching Monticuhpora. Here is a straight, chambered shell, 
possessing constant and similar characters in most of the species ; 
existing in extraordinary abundance at many different horizons ; 
appearing first in the Calciferous, and increasing suddenly in 
numbers in the Trenton period ; progressing, as it were, by leaps ; 
occurring sparingly in one formation and abundantly in the next ; 
and finally dying out altogether in the Permian. As many as 
354 species have been named and described from America by 
authors, though the number of really good species will fall con- 
siderably below this. 
In this genus, too, there have been two methods of procedure. In 
the one case a reduction of certain so-called genera to the rank of 
sub-genera or of synonyms ; and in the other a great multiplication 
of genera. Professor Hall, for example, following Barrande, gives 
seven-teen synonyms for Orthoceras, and besides recognizes three 
sub-genera. Professor Hyatt, on the other hand, has restored many 
names discarded by Barrande and Hall, and has even increased 
their number. He recognizes eighteen distinct genera. These 
are separated upon the external markings of the shell, upon the 
condition of the septa, the appearance of the siphon, and the form 
of the aperture. These straight forms may represent the embry- 
onic stages and ancestral types of the Nautiloid and Ammonitic 
forms, which in later geologic times became the predominant types. 
It is not the purpose here to discuss the validity of the many- 
genera adopted by Professor Hyatt, but it does not appear philo- 
sophical to establish genera upon embryonic characters, especial- 
ly among early Palaeozoic types. The period cannot be regarded 
but as a formative one ; the structurual features of many classes 
had scarcely attained sufficient stability to be constant ; individual 
variation must have been wider than at present ; and with our 
known imperfection of material, to attempt to separate into species 
even, to say nothing of erecting into genera, many of the fossils 
from the older formations, is often a hazardous task. These re- 
